Very accurate or spectrally pure signals are needed in many applications in modern integrated circuit design. The spectral purity and the timing accuracy are just two ways of looking at the same characteristic of the signal. Spectral purity is a frequency domain measurement and jitter is a time domain measurement, however, these terms are often used interchangeably. One way to produce a low jitter signal on chip is to use an on chip LC (inductor-capacitor) tank oscillator.
The basic oscillator consists of a LC tank that sets the operating frequency of the signal, and an amplifier to make up for signal losses in the LC tank and to drive the signal to-the next stage. The frequency and the phase of the oscillator are maintained with a Phase Locked Loop (PLL), where the phase and frequency of the signal output by the oscillator is compared to an external reference signal. Adjustments to drifts in phase or frequency of the oscillator may be made by the PLL through a voltage control pin. This structure is known as a Voltage Controlled Oscillator (VCO).
Desirable characteristics of the VCO are often wide frequency tuning range, low power dissipation, low phase noise or jitter, low sensitivity to the power supply voltage, stable output voltage, low harmonic content, small physical size, and a relatively simple design.
The frequency of the oscillator may be tuned with a device such as a voltage variable capacitor (varactor), that may be included as part of the capacitance of the tank. The oscillation is started by noise in the amplifier, or the LC tank being amplified by the amplifier, and filtered by the tank to cause an exponentially growing sinusoidal oscillation at the tank frequency.
Oscillation occurs when the amplifier characteristics (gain in units of transconductance−Iout/Vin), in consort with the tank impedance, produce a gain greater than unity. The amplitude of the oscillation is limited either by the amplifier running out of voltage swing room or current drive to the tank. In the interest of maintaining the lowest harmonic content, the least sensitivity to the power supply voltage, and the lowest possible power dissipation practical with on-chip inductor values, the amplitude is usually limited by the available drive current of the amplifier. It can be shown that the equivalent impedance of the tank varies with the square of the frequency. For a given drive current, the amplitude of the steady state sine wave varies with the square of the tuning range. In addition, the effective gain of the loop varies with the square of the tuning range. For wide tuning range VCOs, this causes numerous problems.
For example, if the gain (transconductance) of the amplifier is set high enough to ensure that the oscillation will build up at the lowest frequency, the power dissipation at higher frequencies is higher than desired. Also, the amplitude to frequency conversion process converts amplitude noise either from thermal sources or supply induced to phase noise and jitter.
In addition, the sensitivity of the frequency to the control voltage (Kvco) varies. The range of the variation of capacitance of the varactor is fixed by the design of the device. The range of control voltage that the capacitance variation occurs over is a function of the voltage swing of the sinusoid. This occurs because the varactor is a two terminal device referenced to the tank voltage. The voltage on the varactor is the difference between the control voltage and the instantaneous value of the tank voltage. It's effect on the frequency is the product of highly nonlinear control function of the varactor and the signal integrated over a cycle of the oscillation. Thus a larger swing of the sinusoid causes a smaller Kvco. This conventional design for the control device greatly complicates the design of the PLL due to its impact on the stability of the loop.